With the advance of research on optical fibers, the scope of their commercial application has expanded to not only transmission of information but also other fields including transmission of light energy. An infrared light transmission fiber primarily used for transmission of light energy must be made of a material that suits to transmit the infrared light rays. Materials that are known to meet this requirement are as follows:
(1) silver halides and mixtures thereof; PA1 (2) thallium halides and mixtures thereof; PA1 (3) alkali metal halides or alkaline earth metal halides and mixtures thereof; PA1 (4) chalcogenides; and PA1 (5) mixtures of (1), (2) and (3).
The materials of the groups (1) and (2) above are flexible and can be bent easily at a temperature around room temperatures but unlike silica glass that is a typical material for fibers for optical communication and which bends elastically, these materials bend through plastic deformation and are difficult to return to the original shape. Furthermore, the part that has undergone plastic deformation has microscopic defects that cause increased transmission loss, or it sometimes occurs that the part that has undergone repeated plastic deformations fails. Similar properties are observed in the materials classified into (3), (4) and (5) above, which also have the defect of brittleness which is another factor that limits the practical use of these materials. The strength of these materials is significantly lower than that of silica glass, and its tensile strength is from several tens to several hundreds times less than that of silica glass. Among factors that limit the practical use of these materials are sensitivity to visible and ultraviolet rays, hygroscopicity, low strength and great deformation.